Vehicle drive axles typically include a pair of axle shafts for driving vehicle wheels. A drive axle uses a differential to control input speed and torque to the axle shafts. Under ideal conditions, when the vehicle is driven along a straight path, the wheels will be turning at approximately the same speed and the torque will be equally split between both wheels. When the vehicle negotiates a turn, the outer wheel must travel over a greater distance than the inner wheel. The differential allows the inner wheel to turn at a slower speed than the outer wheel as the vehicle turns.
Often the differential includes a differential locking mechanism. When there are poor road conditions, i.e., slippery or rough surfaced roads, the differential locking mechanism allows maximum wheel and tire traction for improved control. If the differential does not have a differential locking mechanism and one tire is on ice, all of the torque and speed will be transferred to the wheel on ice. Thus, the tire just spins on the ice and the vehicle is prohibited from traveling forward. A differential locking mechanism allows the axle shafts to rotate at the same speed while transferring all available torque to the tire not on the ice.
One type of differential locking mechanism is comprised of an air actuated shift collar that locks a differential housing to the axle shafts. An example of one type of air actuated shift collar is disclosed in WO 2004/068002, which is assigned to the assignee of the present invention. Some disadvantages with an air actuation method are the significant number of components that are required, the possibility of leakage, and increased component wear. The significant number of components that are required to operate this system increase assembly time and overall system cost.
Another type of differential locking mechanism, also disclosed in WO 2004/068002, utilizes an electronically actuated shift collar. An electronic actuator generates an electronic signal to move a shift collar from an unlocked position to a locked position. The electronic actuator comprises a coil that surrounds a portion of the shift collar. The coil is selectively energized to move the shift collar to engage a differential case, which locks the differential. A spring disengages the shift collar from the differential case and moves the shift collar back to the unlocked position when the coil is not energized.
While this type of system is more efficient and cost effective than an air actuation system, there are some disadvantages. For example, in order to ensure a smooth engagement, the coil must be positioned on the shift collar and aligned with the differential case and the axle shaft, which can be difficult during assembly. Thus, there is a need for an electronically actuated locking mechanism that can be more easily assembled onto a differential.